Nowadays cell size adjustment can be obtained by typically changing the beam tilt of the antenna through electro-mechanical actuators that control passive devices performing analogue Radio Frequency (RF) processing. This solution, however, presents many drawbacks, as its beam-shaping capability is poorly versatile.
In order to overcome the limitations of the previous approach, digital beamforming techniques can be applied.
According to classical electromagnetic theory, the shape of the beam radiated by planar or linear array antenna can be written as
            E      _        ⁡          (              r        _            )        =                              E          _                0            ⁡              (                  r          _                )              ⁢          F      ⁡              (                  r          ^                )            
where E0(r) is the electromagnetic field radiated by each antenna element, r is the spatial vector, {circumflex over (r)} is the unity-module vector with direction corresponding to spatial vector r and F({circumflex over (r)}) is the array factor of the antenna. Once the basic radiating element is chosen (E0(r)), the shape of the radiation pattern can be fully controlled by operating on the array factor only.
For a Uniform Linear Array (ULA), composed by equally spaced elements, the array factor has the following expression:
      F    ⁡          (      α      )        =            ∑              n        =        0                    N        -        1              ⁢                  w        n            ⁢              exp        ⁡                  [                      j            ⁢                                                  ⁢                          k              0                        ⁢            nd            ⁢                                                  ⁢                          cos              ⁡                              (                α                )                                              ]                    
where k0=2π/λ is the wave number, λ is the wavelength, d is the inter-element spacing, α is the observation direction and wn=wrn+jwin=|wn|exp (j<wn), which is the n-th feed coefficient or weight of the array, allows full control over the array factor shape (hence the beam shape of the field radiated by the antenna).
Techniques devoted to implementing beam forming can be classified into two main approaches: radio frequency (RF) processing and base band (BB) processing.
If radio frequency (RF), typically analogue, processing is considered, weights are applied through RF components which are able to modify both amplitude (RF amplifiers) and phase (RF phase shifters) of RF signal to/from each radiating element.
Document WO 03/015212 illustrates an active phased array antenna system in which a beam former is operable to process an analogue radio frequency signal or an analogue intermediate frequency signal. Programmable electronic power splitters and phase shifters, operating on analogue signals, are used for controlling both the amplitude and phase of each element of the antenna. Phase shifter in particular, which are implemented as Butler matrices, are quite complex systems, whose realization and integration into base stations or transceiver terminals can be complicated.
On the other hand, if baseband (BB), typically digital, signal processing is considered, beam forming is usually realized by multiplying digitised base-band complex signals at each array element by suitable complex coefficients (both in up-link and down-link). An example of a prior art digital beam forming baseband processing (down-link) is shown in FIG. 1.
In down-link, if a generic n-th array element is considered, the complex envelope signal related to it issBBnw=wn{tilde over (s)}(t)
where {tilde over (s)}(t)=i(t)+jq(t) is the complex envelope of the input signal.
Hence, with reference to the scheme of FIG. 1, base-band digital processing just operates a multiplication 2 of a complex input signal {tilde over (s)}(t) by a complex coefficient wn. Once the signal input to the antenna has been weighted, it follows the standard steps through the down-link radio chain: up-conversion 6 to radio frequency (RF), through an intermediate frequency (IF) conversion 4, and high power amplification, not shown in FIG. 1.
The block diagram in FIG. 1 is also valid for the so-called zero-IF technique where the baseband signal is directly up-converted to RF (f0), assuming that fIF=f0 and fΔ=0.
Digital beam forming techniques applied to base-band signals are illustrated for example in documents U.S. Pat. No. 6,052,085 and US 2002/154687.
The techniques illustrated in the above-mentioned documents, operating on baseband signals, imply a good knowledge of how data corresponding to the base-band signals are organized and dealt with in the processing chain. In fact, usually, and particularly with regard to telecommunication apparatuses, this is a confidential and restricted information of the manufacturer. Moreover, if a remote control has to be implemented, apparatuses of the same manufacturer must be necessarily used.
The Applicant has tackled the problem of efficiently performing beam shaping on the radiation pattern of an array antenna, operating exclusively on digital signals.
The Applicant observes that digital beam-forming techniques are much more efficient and cost-effective than analogue ones.
In view of the above, it is an object of the invention to provide an efficient beam shaping technique which can be applied to digitised intermediate frequency signals.